1. Field of the Invention
The present invention relates to non-stop switching equipment for packet-based communication links, and in particular to the non-stop switching equipment for packet-based communication links in a transmission system for transmitting image data packet by packet using a plurality of systems of a redundant configuration, which is able to transmit packets to the output side by a non-stop switching at a packet level while selecting packets of an authentic system from received packets.
2. Description of the Related Art
In a transmission system that increases the reliability of data communications using a dualized redundant configuration, the same data is transmitted in two systems, and the receiving side selects and receives either one of the data reached from the respective systems. According to such a configuration, even when one system is in failure or is under maintenance, the system can receive data, thereby the reliability of data communications can be increased.
Patent Document 1 describes a non-stop transmission path switching system and a receiving apparatus, which accord phases of data at the receiving side of a redundant configuration comprising on the 0 system and the 1 system. Herein, the phase according of data between the systems is carried out, based on frame number given to data frame that is a transmission unit.
Patent Document 2 describes a packet communications apparatus, a packet communications network, and a line switching method therefore, which are provided with a non-stop line function in which errors and loss of user information do not occur even if a line disturbance occurs in an asynchronization communications network such as, for example, ATM and IP. The transmission packet communications apparatus includes a structure by which marked packets are inserted into a packet sequence at a predetermined cycle. The marked packet includes error check information for groups of data packets located between the corresponding marked packet and a marked packet inserted immediately before, and identification information showing that the corresponding packet is a marked packet. The receiving packet apparatus carries out phase administration and error detection of the corresponding data packets based on the contents of the received marked packet, and selects a system which finally outputted group by group of packets corresponding the marked packet according to the error detection.
Patent Document 3 relates to a fault detection system in a redundant configuration system of an Internet work apparatus, and describes that, in order to switch from the port of a currently used router to a replacement port when the port has a fault occurrence and to improve the reliability of operation after switching, an OAM (operation administration and maintenance) frame is periodically inserted at the transmission side, and the line quality for the respective systems is measured at the receiving side.
Non-Patent Document 1 shows a method for executing phase focusing between systems by a delay insertion, aiming at anon-stop switching at the unit of ATM cell.
[Patent Document 1] Japanese Published Unexamined Patent Application No. 2001-237810
[Patent Document 2] Japanese Published Unexamined Patent Application No. 2003-273902
[Patent Document 3] Japanese Published Unexamined Patent Application No. H10-303966
[Non-Patent Document 1] Technical Report of the Institute of Electronics, Information and Communication Engineers-(IEICE), October 1992, Vol. 92, No. 287, CS92-48
The arts described in Patent Document 1 and Non-Patent Document 1 are based on the assumption that fault information of respective systems which is the current/reserve selection reference is given from the network facility side such as a switchboard, wherein the receiving side apparatus is not provided with a mechanism of fault detection.
As in the arts described in Patent Document 1 and Non-Patent Document 1, in such a configuration that a fault system is detected at the network facility side, and the system is switched to an authentic system at the receiving side apparatus based on the detection, it takes a predetermined duration of time until the systems are switched after the fault is detected, wherein the transmission system is brought into a failure state in the duration of time. In a real-time based data transmission system such as distribution of live images, it is desirable that the system is prevented from being temporarily brought into a failure state. Therefore, in view of securing the reliability using a redundant configuration, it is desired that selection of an authentic system and a system switching are carried out at the same point at the receiving side.
Since, in the arts described in Patent Documents 2 and 3, it is necessary to additionally generate packets for quality measurement periodically at the transmission side in order to detect errors in the respective systems, there are problems (1) through (3) as shown below.
(1) Large-scaled function enhancement is required at the transmission side apparatus when carrying out a system configuration corresponding to non-stop switching.
(2) Since it becomes necessary to transmit packets for quality measurement other than data packets in a transmission line, the transmission efficiency is lowered. In addition, since no consideration is given to losses of the packets for quality measurement where the transmission line is interrupted, sufficient reliability is not secured in measurement of the quality of each of the systems.(3) Since a processing delay occurs depending on an insertion cycle of packets for quality measurement and quality measurement time at the receiving side, these arts are not suitable for a data transmission system that requires low delay.